* Prices may differ from that shown
I was reminded of how useful this small, cheap thermometer actually is just the other day when, unexpectedly on the coldest day of the year, we were left with no power for an entire day and no way of heating our tropical aquarium. By dint of putting a duvet over the aquarium and just checking the thermometer was at a satisfactory temperature (most tropical fish need the water to be somewhere between 24-27 degrees centigrade), we were able to monitor the water and avoid any casualties. On a day to day basis this thermometer is quite simply a vital accessory in my opinion for any fishkeeper to have, allowing you to quickly check all is well with the heater and your fish, and far better in my opinion than stick on strip alternatives which may not give you an accurate reading.
The thermometer is called a floating thermometer as, in theory, you can leave it to float around your tank. I prefer to use the discrete see through suction cup to stick it to the side wall of my tank, well away from the heater. It's 11cm or so tall and as it is made of glass is quite unnoticeable once in place. I've never, in two years of using this had the suction cup come unstuck, though the thermometer itself does get algae ridden over time it's easily cleaned and stuck back where it continues giving what I would assume to be accurate readings - in any case we have never had an issue with the water temperature. The gauge, which is in centigrade and fahrenheit is clear to read and, most usefully, has a green section between 21-27 degrees so that you can see at a glance if all is well and that there has been, for example, no heater failure.
The contents of the inner tube are red so show up well when you are trying to see what temperature your aquarium is at. I don't anticipate breaking this item in my tank but if I were to it's good to know that the thermometer is non-mercury - mercury is very toxic to fish, there are some little lead balls weighing down the bottom of the thermometer which would need removing from the tank if the thermometer were to break open; my fish seem to be small enough not to cause this kind of issue and mainly ignore the thermometer totally.
The only criticism I would make of this item is that the scale and numbers are a bit small but not so small that on set up I wasn't able to adjust my heater and be sure of the exact temperature my water would be maintained at, something which can be important depending on the type of fish you are keeping clearly.
I paid a couple of pounds for this thermometer when I bought my tank - you can currently buy one for £2.49 from petsathome. There's no maintenance required with this item, or batteries like a digital thermometer would require and it just does the job it is designed to well and with no fuss. On that basis I recommend it.
A thermometer is pretty standard kit for anyone who keeps tropical fish.
I happen to be pretty good and guessing if the water is a safe temperature - but pretty good does not cut it - one wrong guess and my fish could suffer needlessly or possibly die.
For the most part, aquarium thermometers are very much the same. Modern ones will be made of plastic rather than glass to prevent disaster should one break. They will also have a coloured alcohol, not mercury which is an extremely toxic substance even in small amounts. Virtually all of them will have temperatures in fahrenheit on one side and Celsius on the other with a narrow green band indicating the desired temperature range, and some small metal balls in the bottom of the thermometer. I have found all the internal tank thermometers I have used very accurate, so there is no choosing between brands for this feature either.
So why do I recommend this one? Well, I find everything else equal, but this thermometer has excelled in one department for me. It stays on the tank wall! No floating about and terrifying my fish, no banging up against the side of the tank constantly, and of course it is much easier to read properly situated against the glass on the far side of the tank from the heater. Since I find every other feature identical, and price range is also similar ( I think I paid about £2) I would much much rather have the one that stays put! As an added bonus I prefer the clear suction cup which is less noticeable than the black, and find the loop it attaches with a better idea than just pushing the suction cup on the back of thermometer like you do with most.
A FEW NOTES ON THERMOMETERS
Always place your thermometer away from the heater. You will get an inaccurate reading right next to the heater.
If you think the thermometer is wrong, get another thermometer and double check. While they are usually very reliable, the correct temperature is so important to your fishes health, better safe than sorry.
Never use an old fashioned glass thermometer or any thermometer not specifically made for aquariums. A very tiny leak could easily poison your fish long before you notice it.
A thermometer is absolutely useless unless you read it - make it practice to quickly check temperature when feeding your fish.